Animals which live in an aquatic environment have certain basic requirements for proper growth, such as an optimum water temperature range, photoperiod preference, salinity range, and feed preference. In addition to these basic requirements, certain aquatic animals, such as abalone, sea urchins, and periwinkles, require a surface on which to attach, and feed by roaming across the attached surface in search of food substances, for example micro algae and macro algae.
The situation is further complicated by the different needs and feeding patterns of different species of aquatic animal. Some species of animals roam until they find the feed, other species will wait until a piece of seaweed drifts by. Current methods for the farming of such grazing aquatic animals attempt to provide a support onto which the animals can attach and additionally provide a food substrate.
In the simplest of these methods, the animals are immersed in a bucket or tank of water, usually within an inner mesh cage to permit ease of collection and removal. Feed substrate is provided by means of adding sheets of macroalgae, for example seaweed (e.g. the brown macroalgae, Laminaria spp) to the tank. The animals attach onto the seaweed layers.
This method has a number of disadvantages; including, the seaweed and animals are in the water column and therefore obstruct the flow of water; collection of the animals is awkward due to their being irregularly scattered about a disorganised mass of seaweed; replacing old or rotting feed substrate is made difficult by the fact that animals are randomly adhered to the feed substrate; it is impractical to culture more than one species per tank or different ages of the same species as these will have to be sorted upon harvesting.
JP 2000/175,591 discloses a method of aqua culturing abalone and sea urchins in open sea. The marine creatures are provided with artificial and natural seaweed upon which to attach; set upon a grid system for growing the seaweed. This method, while avoiding the disadvantage of obstructed water flow, retains the remaining above disadvantages.
In an effort to provide additional support for the animals, alternative methods of aquaculture are used. These provide solid surfaces for the animals to adhere to; in the simplest embodiment, these take the form of corrugated plastic sheets positioned perpendicularly in a tank system. Feed is simply positioned between the corrugated plastic sheets. The animals attach onto the plastic sheeting and feed when some the feed comes in contact with the plastic sheet. Animals show a preference for attaching to a stable solid surface rather than to seaweed.
U.S. Pat. No. 4,253,418 discloses a method of abalone mariculture consisting of a tank with a series of inserts designed to greatly increase the available surface area for the animals to adhere to. These inserts are of the form of a plurality of grid sheets providing a multiplicity of intersecting surfaces suspended approximately vertically in a tank of water. However, these methods are inefficient as the animals attached to the solid surfaces need to roam continuously to acquire food and therefore consume energy, resulting in reduced size to feed ratios and also requiring a greater surface area per animal. Therefore, this type of method, while providing greater surface areas to attach to still retains some of the disadvantages attendant with the method of simply culturing the animals in a tank with free floating feed.
Therefore, there are several major disadvantages to the use of such growing systems. Most importantly, the feed (for example, sheets of macro algae) is in the water column while the animals are attached to the support surface. Therefore, the only feed that is accessible to the animals is the minority of feed which contacts the support surface. In aquaculture terms, the current systems for growing grazing animals have a low feed availability, as the majority of the feed is in the water column and does not contact the support surface.
Currently, grazing aquatic animals are fed freshly-harvested macroalgae, for example, fronds of the brown algae Laminaria spp. Such freshly harvested feed has a ‘shelf life’ of only several days before it begins to deteriorate and becomes unacceptable to the animals. Since in the current growing system a certain proportion of the feed is unavailable to the animals, a certain proportion of the feed will remained unconsumed until it begins to decompose and be unacceptable. Therefore, another disadvantage of the current growing methods is the uneconomical use of feed.
Moreover, in the current ongrowing systems the animals must wander on the support surface in search of feed contacting the surface. As a certain amount of surface area has to be provided per animal, the current ongrowing systems require a large surface area. In aquaculture terms, the current ongrowing systems have a low stocking density (i.e. the number of animals per unit area or volume). This requirement for a large surface area (low stocking density) is expensive and uneconomical both in terms of construction and running costs.
A further disadvantage of the current systems is the obstruction of water flow by the feed positioned in the water column. In all aquaculture systems a certain amount of water flow is required to provide oxygen and remove wastes. In the current ongrowing systems for grazing aquatic animals, the positioning of feed in the water column obstructs this water flow. This obstruction of the water flow by the feed results in lower oxygen and higher waste concentrations, and hence non-optimum growing conditions which may result in a lower growth rate for the animals.
To compensate for the obstruction of water flow by the feed, it is possible to increase the flow rate of the water by increasing pump flow (by using larger or additional water pumps). By increasing water flow though, construction and running costs (e.g. purchase and electricity for larger/additional pumps) are also increased. Furthermore, the higher water flow rate will cause faster ‘mixing’ of the feed in the water column. The feed will therefore have a shorter contact time with the support surface onto which the animals are attached and feed availability (and hence growth rate) will decrease.
JP 10,276,607 discloses a method of feeding artificial feed to sea urchins. This method comprises a solid card of artificial feed held vertically in place by a flange. One of the disadvantages associated with this method is that it relies exclusively on an artificial feed substrate. A further disadvantage is that the feed substrate may deteriorate over immersion time, and as it is consumed by the animals, resulting in a decrease in the security of its lodgement in the supporting structure.
There is thus a requirement for a growing system for grazing aquatic animals which allows for a high stocking density and provides for maximum feed availability with minimum obstruction to water flow.